1. Field of the Invention
This invention relates to the field of subsurface exploration and, more particularly, to logging techniques for detecting and locating fractures in earth formations.
2. Description of Related Art
Electromagnetic (EM) logging tools have been employed in the field of subsurface exploration for many years. These logging tools or instruments entail an elongated support equipped with antennas that are operable as sources or sensors. The antennas on these tools are generally formed as loops or coils of conductive wire. In operation, a transmitter antenna is energized by an alternating current to emit EM energy through the borehole fluid (“mud”) and into the surrounding formations. The emitted energy interacts with the borehole and formation to produce signals that are detected and measured by one or more receiver antennas. The detected signals reflect the interaction with the mud and the formation. By processing the detected signal data, a log or profile of the formation and/or borehole properties is determined.
Conventional EM logging techniques include “wireline” logging and logging-while-drilling (LWD) or measurement-while-drilling (MWD). Wireline logging entails lowering the instrument into the borehole at the end of an electrical cable to obtain the subsurface measurements as the instrument is moved along the borehole. LWD/MWD entails attaching the instrument disposed in a drill collar to a drilling assembly while a borehole is being drilled through earth formations. A developing method, sometimes referred to as logging-while-tripping (LWT), involves sending a small diameter “run-in” tool through the drill pipe to measure the downhole properties as the drill string is extracted or tripped out of the hole.
A coil or loop-type antenna carrying a current can be represented as a magnetic dipole having a magnetic moment strength proportional to the product of the current and the area encompassed by the coil. The magnetic moment direction can be represented by a vector perpendicular to the plane of the coil. In the case of more complicated coils, which do not lie in a single plane (e.g. saddle coils as described in published U.S. patent application Ser. No. 20010004212 A1, published Jun. 21, 2001), the direction of the dipole moment is given by: ×dl and is perpendicular to the effective area of the coil. This integral relates to the standard definition of a magnetic dipole of a circuit. See J. A. Stratton, ELECTROMAGNETIC THEORY, McGraw Hill, N.Y., 1941, p. 235, FIG. 41. Integration is over the contour that defines the coil, r is the position vector and dl is the differential segment of the contour.
In conventional EM logging tools, the transmitter and receiver antennas are typically mounted with their axes along, or parallel, to the longitudinal axis of the tool. Thus, these instruments are implemented with antennas having longitudinal magnetic dipoles (LMD). An emerging technique in the field of well logging is the use of tools with tilted or transverse antennas, i.e., where the antenna's axis is not parallel to the support axis. These tools are thus implemented with antennas having a transverse or tilted magnetic dipole moment (TMD). One logging tool configuration comprises triaxial antennas, involving three coils with magnetic moments that are not co-planar. The aim of these TMD configurations is to provide EM measurements with directed sensitivity. Logging tools equipped with TMDs are described in U.S. Pat. Nos. 6,044,325, 4,319,191, 5,115,198, 5,508,616, 5,757,191, 5,781,436 and 6,147,496.
EM propagation tools measure the resistivity (or conductivity) of the formation by transmitting radio frequency signals into the formation and using spaced-apart receivers to measure the relative amplitude and phase of the detected EM signals. These tools transmit the EM energy at a frequency in the range of about 0.1 to 10 MHz. A propagation tool typically has two or more receivers disposed at different distances from the transmitter(s). The signals reaching the receivers travel different distances and are attenuated to different extents and are phase-shifted to different extents. In analysis, the detected signals are processed to derive a magnitude ratio (attenuation) and phase difference (phase shift). The attenuation and phase shift of the signals are indicative of the conductivity of the formation. U.S. Pat. Nos. 4,899,112 and 4,968,940 describe conventional propagation tools and signal processing.
In addition to the formation resistivity, identification of subsurface fractures is important in hydrocarbon exploration and production. Fractures are cracks or breakages within the rocks or formations. Fractures can enhance permeability of rocks or earth formations by connecting pores in the formations. Fractures may be filled with formation fluids, either brine or hydrocarbons. If a fracture is filled with hydrocarbons, it will be less conductive, i.e., a resistive fracture. Wells drilled perpendicularly to resistive fractures tend to be more “productive” (i.e., produce lager quantities of hydrocarbons). Thus, the determination of a resistive fracture's orientation may help improve oil and gas production. In addition, the orientation of a fracture provides the direction of principal stress, which affects the stability of the well and it helps in predicting which well trajectory will be the most stable. Knowledge of fracture orientations also aids in the prediction of fracture strengths of the earth formation. Furthermore, the presence of fractures may indicate that the mud weight used for drilling the well is too high so as to cause fracture of the rock.
Methods and systems have been developed for detecting fractures and determining their orientation. For example, U.S. Pat. No. 3,668,619 describes the rotation of a logging tool having a single acoustic transducer that senses the reflected acoustic energy to detect fractures. U.S. Pat. No. 5,121,363 describes a method for locating a subsurface fracture based on an orbital vibrator equipped with two orthogonal motion sensors and an orientation detector. U.S. Pat. No. 4,802,144 uses the measurement of hydraulic impedance to determine fractures. U.S. Pat. No. 2,244,484 measures downhole impedance to locate fractures by determining propagation velocity.
There remains a need for improved techniques for detecting and locating fractures, and for determining their orientations, particularly using propagation-type tools.